The pathways of mammalian cysteine metabolism are numerous and complex. At least 1 distinct enzymes active toward cysteine have been identified, and the pathways initiate by these enzymes have been shown to lead to a multitude of intermediate products. Many of the pathways eventually intersect to either regenerate cysteine or to form a small number of catabolic products (taurine, sulfate, pyruvate). The multiplicity of pathway and the absence of unique products has impeded attempts to determine quantitatively the importance of individual pathways in the overall metabolism of cysteine. Accurate interpretation of pulse-labeling studies is seriously compromised, for example, by cysteine recycling, pathway convergence, and rapid inter-organ transport of cysteine metabolites. Studies of cysteine metabolism would be greatly facilitated by the availability and use of enzyme-specific inhibitors that allow selected pathways to be blocked in vivo and by the development of cysteine analogs that are subject to only limited metabolism. By decreasing the number of metabolic pathways requiring consideration these techniques substantially simplify the interpretation of in vivo experiments Inhibitors of specific pathways and enzymes are also of potential use in the therapeutic manipulation of cysteine metabolism; applications are indicated in the sensitization of tumors to chemotherapeutic radiation and immunological killing, in specifically decreasing cysteine availability to tumors, in regulating the metabolism of leukotrienes, and in the sensitization of parasites to oxidative stress. The elucidation of mammalian cysteine metabolism through the design, synthesis and use of modified substrates and enzyme- specific inhibitors is the overall objective of the studies proposed. Inhibitors and substrates will be tested for activity and specificity with isolated enzymes, and will then be used in vivo and in vitro to characterize cysteine metabolism. Four specific objectives are addressed: (1) the partitioning of cysteine among several catabolic pathways will be determined in vivo and in tissue preparations. (2) Specific inhibitors of the phosphopantetheine synthesis will be made and used to control taurine formation via the cysteamine pathway. (3) Hypotaurine oxidation will be characterized in vivo to provide criteria of specificity and rate that can guide in vitro studies. (4) New inhibitors of glutathione (GSH) synthesis and degradation will be developed and used to study the protective role and transport of GHS.